The hormonal action of abscisic acid (ABA) in plants is controlled by the precise balance between its biosynthesis and catabolism. In plants, ABA 8 0 -hydroxylation is thought to play a predominant role in ABA catabolism. ABA 8 0 -hydroxylase was shown to be a cytochrome P450 (P450); however, its corresponding gene had not been identified. Through phylogenetic and DNA microarray analyses during seed imbibition, the candidate genes for this enzyme were narrowed down from 272 Arabidopsis P450 genes. These candidate genes were functionally expressed in yeast to reveal that members of the CYP707A family, CYP707A1-CYP707A4, encode ABA 8 0 -hydroxylases. Expression analyses revealed that CYP707A2 is responsible for the rapid decrease in ABA level during seed imbibition. During drought stress conditions, all CYP707A genes were upregulated, and upon rehydration a significant increase in mRNA level was observed. Consistent with the expression analyses, cyp707a2 mutants exhibited hyperdormancy in seeds and accumulated six-fold greater ABA content than wild type. These results demonstrate that CYP707A family genes play a major regulatory role in controlling the level of ABA in plants.

Glc has hormone-like functions and controls many vital processes through mostly unknown mechanisms in plants. We report here on the molecular cloning of GLUCOSE INSENSITIVE1 (GIN1) and ABSCISIC ACID DEFICIENT2 (ABA2) which encodes a unique Arabidopsis short-chain dehydrogenase/reductase (SDR1) that functions as a molecular link between nutrient signaling and plant hormone biosynthesis. SDR1 is related to SDR superfamily members involved in retinoid and steroid hormone biosynthesis in mammals and sex determination in maize. Glc antagonizes ethylene signaling by activating ABA2/GIN1 and other abscisic acid (ABA) biosynthesis and signaling genes, which requires Glc and ABA synergistically. Analyses of aba2/gin1 null mutants define dual functions of endogenous ABA in inhibiting the postgermination developmental switch modulated by distinct Glc and osmotic signals and in promoting organ and body size and fertility in the absence of severe stress. SDR1 is sufficient for the multistep conversion of plastid- and carotenoid-derived xanthoxin to abscisic aldehyde in the cytosol. The surprisingly restricted spatial and temporal expression of SDR1 suggests the dynamic mobilization of ABA precursors and/or ABA.

SummaryTo reveal the transcriptomes of Arabidopsis seed, comprehensive expression analysis was performed using ATH1 GeneChips (Affymetrix, Santa Clara, CA, USA). In the dry seed, more than 12 000 stored mRNA species were detected, including all ontological categories. Statistical analysis revealed that promoters of highly expressed genes in wild-type dry seeds overrepresented abscisic acid-responsive elements (ABREs) containing the core motif ACGT. Although the coupling element and seed-specific enhancer RY motif alone were not prominently overrepresented in genes with high expression, the presence of these elements in combination with ABRE was associated with particularly high gene expression. The transcriptome of the imbibed seeds differed from that of the dry seed even at 6 h after seed imbibition. After imbibition many upregulated and downregulated genes were co-regulated in clusters of three to five genes. Genes for which expression was affected by the abi5 mutation tended to be located in clusters, suggesting that transactivation by ABI5 is not restricted to a single gene, but affects other proximal genes. Furthermore, cytosine methylation was observed not only in large silent retrotransposon clusters in centromeric regions, but also in non-centromeric silent gene clusters in the seed. These results suggest that such regions might be transcriptionally silenced by methylation or heterochromatin structures. Our analyses reveal that transcriptomes of Arabidopsis seed are characterized by multiple regulatory mechanisms: epigenetic chromatin structures, chromosomal locations (e.g. co-regulated gene clusters) and cis-acting elements.

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Endogenous abscisic acid (ABA) levels are regulated by both biosynthesis and catabolism of the hormone. ABA 8#-hydroxylase is considered to be the key catabolic enzyme in many physiological processes. We have previously identified that four members of the Arabidopsis (Arabidopsis thaliana) CYP707A gene family (CYP707A1 to CYP707A4) encode ABA 8#-hydroxylases, and that the cyp707a2 mutants showed an increase in ABA levels in dry and imbibed seeds. In this study, we showed that the cyp707a1 mutant accumulated ABA to higher levels in dry seeds than the cyp707a2 mutant. Expression analysis showed that the CYP707A1 was expressed predominantly during mid-maturation and was down-regulated during latematuration. Concomitantly, the CYP707A2 transcript levels increased from late-maturation to mature dry seed. Phenotypic analysis of single and double cyp707a mutants indicates that the CYP707A1 is important for reducing ABA levels during midmaturation. On the other hand, CYP707A2 is responsible for the regulation of ABA levels from late-maturation to germination. Moreover, CYP707A1 and CYP707A3 were also shown to be involved in postgermination growth. Spatial expression analysis suggests that CYP707A1 was expressed predominantly in embryo during mid-maturation, whereas CYP707A2 expression was detected in both embryo and endosperm from late-maturation to germination. Our results demonstrate that each CYP707A gene plays a distinct role during seed development and postgermination growth.